[37] In the 1930s the
wind tunnel evolutionary tree had split into two main
branches:

1. The branch concerned with scale
effects and the reach toward the higher Reynolds numbers
characteristic of actual flight. The Langley tunnel species growing
on this branch were the variable density tunnel, the full-scale
tunnel, and the 19-foot pressure tunnel.

2. The branch dealing with high-speed effects,
as represented by the 24-inch high-speed tunnel and the 8-foot
high-speed tunnel.

A third branch sprouted unexpectedly in the
late 1930s when Eastman Jacobs and his associates at...

An early version of the X-2
research aircraft undergoes tests on the transonic "bump" of the
Langley high-speed 7 x 10-foot wind tunnel. Local airflow over the
bump reaches the speed of sound.

....Langley were assessing the performance of
wings developed in the VDT. For some unexplained reason, the wings
usually performed better in actual flight than wind tunnel tests had
predicted- a strange turnabout because one generally expected
laboratory tests to be more optimistic that flight results. Careful
research demonstrated that the performance gap was due to undetected
turbulence in Langley's wind tunnels. The atmosphere outdoors was
actually quieter and more homogeneous than that in the best wind
tunnels.

In contrast to wind gusts and other
large-scale turbulence in the atmosphere, the wind tunnel's fans and
air-guiding structures induced fine-scale random fluctuations in
local air velocity and flow angle. This microscopic "weather"
disturbed the thin boundary layer of air next to the surface of the
wind tunnel models. Lift, drag, and other measurements were
compromised in ways that could not be corrected for.

Wind tunnel designers employ two techniques to
tranquilize microscopic air turbulence. In the first, the airstream
is simply squeezed into a duct with a much smaller cross-sectional
area. In effect, the squeezing or contraction irons out some of the
disorderly [39] airflow-an aerodynamic mangle, as it were. Modern
low-turbulence tunnels usually have a contraction section in which
the flow area is reduced by a factor of 15 or more. The second
technique uses a settling or stilling chamber upstream of the
contraction section. In this chamber, baffles and screens (some with
wire as thin as human hairs) smooth out the flow by breaking up the
eddies.

Although these two principles were recognized
in the late 1930s by the NACA engineers contemplating their first
low-turbulence tunnel, no one had ever built a large tunnel using
high contraction ratios in combination with a settling chamber packed
with honeycomb and fine-meshed screens. Would such a tunnel work at
the high Reynolds numbers demanded? Before investing in a full-scale,
pressurized tunnel of such novel design, it seemed wise to build a
cheap model to work out any unexpected engineering problems that
might arise.

At this period (the late 1930s), the
desirability of low turbulence in wind tunnels was not widely
appreciated. Funds for a "low-turbulence" tunnel would have been
difficult to justify. Aircraft icing, however, was a "hot" topic. The
model of the low-turbulence tunnel was therefore designated the "
NACA Ice Tunnel." Fabricated from plywood with an inner lining of
sheet metal, the ice tunnel was completed in

April 1938. The contraction ratio was 19.6 to
1, with a test section 7.5 feet high and 3 feet wide. Airspeed was
155 mph. True to its announced purpose, the tunnel walls were
insulated with a thick wrapping of crude insulation, and
refrigerating equipment of sorts was added. This consisted simply of
an open tank of ethylene glycol cooled by blocks of dry ice, with the
cold mixture pumped through coils that cooled air drawn from the
tunnel. Ice actually did form on the leading edge of an airfoil
during one of the early, rather perfunctory tests, and the ice tunnel
fulfilled its announced purpose.

By October 1940, however, aircraft icing had
been forgotten and an array of honeycomb and screening had been
installed upstream of the test section. As the tunnel designers had
hoped, the air in the test section was almost devoid of turbulence,
and a new horizon for aerodynamic research was opened.

The plywood and tin model did its job well.
Not only was it employed to perform useful research in its own right,
but it also served as a design base for a more permanent facility-the
so-called Low Turbulence Pressure Tunnel (LTPT). In the LTPT, a heavy
steel shell replaced the flimsy plywood and tin because the tunnel
was to be pressurized to 10 atmospheres. The test section was 7.5 x 3
feet. The contraction ratio was a bit smaller than the
model...

Schematic of the Langley two-dimensional,
low-turbulence tunnel, also known as the ice tunnel.

[40] Phantom drawing of the Langley two-dimensional,
low-turbulence pressure tunnel (LTPT) in foreground The ice tunnel ZS
in the background.

...(17.6 to 1), but 11 screening elements were
installed so that the turbulence levels approached those encountered
in the natural atmosphere.

When the LTPT commenced operation in the
spring of 1941, it began war work on a crash basis. With its unique
low-turbulence-flow characteristics, it was an ideal tool with which
to explore the capabilities of a revolutionary type of wing-the newly
conceived laminar-flow airfoil. The practical consequences of the new
wing were far reaching and of utmost importance in the war effort. It
was "fortunate" that Langley engineers, via their ice tunnel, had
just the right instrument on hand at the right time.

What was behind this prescience? As noted
earlier, Osborne Reynolds had demonstrated in 1883 two types of fluid
flow in his classic pipe-flow experiments. The first, turbulent flow,
is characterized by high skin friction, which duly translates into
high aircraft drag. The second, laminar flow, occurs when the layers
of air slide smoothly over one another without breaking up into
swirls and eddies. Skin friction in laminar flow is very
low-typically one-fifth of that in turbulent flow. If the airflow
over a fighter or bomber wing could be made mostly laminar, its range
could be increased markedly because less fuel would be expended in
fighting drag. The low- turbulence pressure tunnel was made to order
to explore laminar flow because its airflow was so quiet and smooth
that the layers of air sliding over the test wings were not disturbed
by tunnel- induced turbulence.

Eastman Jacobs and his associates at Langley
knew that the laminar flow of air over a wing was inherently unstable
and that it broke up into turbulence just beyond the leading edge of
the wing. However, Reynolds, Prandtl, and other aerodynamic theorists
had predicted that if the layer of air closest to the wing surface
(the boundary layer) was moving into a region of decreasing pressure,
the laminar nature of the flow could be stabilized. Pursuing this
lead with the earlier ice tunnel and the new LTPT, they developed a
whole new series of laminar-flow airfoils. These, when translated
into practical wings, had the potential for greatly reduced drag
compared to the old wings with fully turbulent boundary layers. Ames
aerodynamicists used their 1 x 3.5-foot tunnel to refine...

[41] A wake-survey rake in the Langley ice tunnel measures
air pressures with a series of manometers. (A) Pressures across the
wake at zero lift; the hump is proportional to airfoil drag. (B) When
airflow is laminar, the drag is reduced.

A
P-51 at Langley in December 1951. The great range of this fighter was
credited to the new NACA 6-genes, low drag wing, developed in NACA 's
low-turbulence tunnel. With less fuel needed to fight drag, the P-5 1
could escort bombers all the way to Berlin, drastically cutting
bomber losses. (Photo courtesy EAA Air Museum
Foundation)

...airfoil contours and establish performance
characteristics in the transonic range. The best of the new
laminar-flow, low-drag airfoils (called the "6-series") was quickly
adopted by the designers of advanced World War II fighters and
bombers. This airfoil family still contributes to wing design in
today's subsonic jets and propeller-driven aircraft.